Nanotechnology-Enabled Sensors

434 Chapter 7: Organic Nanotechnology Enabled Sensors
is located (e.g. cytoplasmic polyamines and Mg 2+ , which act on many eukaryotic
channels), natural but exogenous (e.g. plant and animal toxins), or
synthetic (e.g. various therapeutic agents). 126 Channels can exist in opened
and closed states, and the presence of a channel blocker reduces (partial
block) or eliminates (full block) the conductance of an open state.
This modulated current has high information content. In particular, the
modulated current reveals both the identity and the concentration of a
blocker, which allows a channel protein to be used as a stochastic sensing
element. The identity of the blocker depends on the nature of the blocking
events, e.g. the dwelling time, the extent of blockage, and their voltage
dependences.
Current (pA)
Pore opened
Pore closed
Time (ms)
Fig. 7.53 The opening and closing of the pore changes the current in the patchand-clamp
set-up.
Another elegant approach for the development of biosensors using selective
membrane proteins was proposed by Cornel et al. 129 In their work
the detection of analytes possessing multiple recognition sites was
performed using the structure shown schematically in Fig. 7.54. This
structure was assembled using a combination of sulphur–gold chemistry
and physisorption as described in Fig. 7.54. The membrane consists of
lipids and channels, some of which are immobilized on the gold surface
and some of which diffuse laterally within the plane of the membrane. The
antibodies on the mobile channels scan an area of the order of 1 μm 2 in
less than 5 minutes. As a result, with a low density of channels and a high
density of immobilized antibodies, each channel can access up to 10 3 times
more captured antibodies than if the gating mechanism were triggered by a
directing binding of the analyte to the channels. The speed and sensitivity